The exceptional nature of Stephenson 2-18

Stephenson 2-18 is the brightest star visible in the center of this image. At first glance it may seem like a star like many others that almost gets lost in this rich star field, but in-depth analyzes of its properties have revealed its exceptional nature.

According to the latest studies, the star should in fact have a diameter of 2150 solar diameters: thus it would be the largest star discovered so far!

If it were located inside the Solar System, its photosphere would reach the orbit of Saturn!

Stephenson 2-18 was therefore classified as a red hypergiant with a brightness 440,000 times greater than that of the Sun and a surface temperature of 3200 degrees Kelvin.

Stephenson 2-18 is located at a distance of 20,000 light years from Earth, in the direction of the constellation Scutum.

Despite being so bright, the star cannot be seen with the naked eye. It is in fact in the same direction as the Milky Way’s band: all the visible light emitted by the star is therefore absorbed by the dust that lies along the line of sight. Stephenson 2-18 is therefore observed in the infrared wavelengths, which are not affected by the presence of dust.

The existence of a star such as Stephenson 2-18 puts in serious crisis the theoretical models developed up to now, according to which a red giant cannot expand to such a large diameter. To answer these questions, further observations will be needed, which perhaps will reduce the radius of the star, bringing it back within the limits set by theoretical models.

Credit: Université de Strasbourg/CNRS.